Typically, four-color inkjet printers have replaceable ink cartridges providing cyan (C), yellow (Y), magenta (M) and black (K) ink printing. Four separate color cartridges are provided, rather than providing them in a mono-block configuration, typically to increase yield in manufacture. Precise alignment among the various ink cartridges, or pens, is required to produce high quality print without noticeable dot misregistration, color variegation or other undesirable visual effects. For example, in order to print a dark cyan line, a linear array of cyan and black dots must be placed precisely on top of one another. Otherwise, the resulting pattern would appear as two parallel lines of cyan and black. Such slight misalignment, or misregistration, between two or more ink pens could be adjusted for by a shift of the image to be printed as between the two colors prior to printing. Thus, in a four-color printer wherein, as is typical, a black ink pen and three color ink pens are provided in the form of separate, changeable pens or cartridges, alignment between the independent, and possibly slightly misaligned, pens is required. Such inter-pen or inter-color misalignment of course is not limited to the case where the various pens are physically separate, as misalignment may result from dimensional tolerances in the manufacture of, for example, a mono-block printhead having two or more integrated ink cartridges and associated ink droplet outlets or orifices.
Interactive techniques for making alignment adjustments have been developed whereby an inkjet printer's controller causes plural, progressive alignment target patterns, e.g. nominally aligned black and adjacent primary colored line segments, to be printed and the operator chooses the best alignment pattern and enters such a choice into the printer controller's memory, whereby the printer uses such stored alignment data thereafter to properly align images produced by the slightly misaligned pens. Such a technique is described in European Application number 93307586.3 entitled PEN ALIGNMENT METHOD AND APPARATUS FOR PLURAL PEN INK-JET PRINTHEAD CARRIAGE, which was filed Sep. 24, 1993 claiming priority of U.S. patent application Ser. No. 951,067 filed Sep. 25, 1992, which was published as EP Publication number 0 589 718 A1 on Mar. 30, 1994, which was granted Jan. 8, 1997 as European Patent No. EP 0589718, and which is commonly assigned herewith. Familiarity with the disclosure of that patent is assumed. That patent disclosure does not address the problem of inter-pen alignment as between yellow and another color of ink whereby a yellow target alignment pattern printed on a white print medium may be invisible or virtually invisible to the operator.
A method for rendering such an "invisible" ink image capable of being sensed by an opto-electronic sensor in an ink-jet printer is described in U.S. patent application Ser. No. 08/636,439 entitled SYSTEM AND METHOD FOR DETERMINING PRESENCE OF INKS THAT ARE INVISIBLE TO SENSING DEVICES, which was filed Apr. 22, 1996, and which is commonly assigned herewith. That application describes an automatic process for relatively high-performance and -cost printers whereby a fractional fill pattern is produced using visible ink, immediately followed by a fractional fill pattern within the same region produced by using an invisible ink, e.g. yellow. Ink bleeding within the region, as between the visible and invisible inks, produces a relatively more solid fill, thereby rendering the pattern capable of being detected by a built-in optical sensor. The application does not suggest inexpensive, semi-automatic, user-interactive inter-pen alignment.